1. Field of the Invention
The present invention relates to an electrostatic pressure-gradient microphone capsule to be mounted essentially flush in or behind especially flat mounting surfaces. The microphone capsule includes a capsule housing with two sound inlet openings which may be divided, a diaphragm tightly mounted on a diaphragm ring, an electrode and possibly an acoustic friction.
2. Description of the Related Art
Independently of their physical manner of operation, electrostatic transducers for microphones have a diaphragm which is subjected to the field of sound and is excited to vibrations by the field of sound. Consequently, the invention is directed to an electrostatic microphone.
The electrodes of the electrostatic transducer are an elastic, tightly mounted diaphragm and a rigid electrode which is usually just called electrode. Both electrodes form a capacitor whose electrical capacity changes as a result of pressure variations of the field of sound. Since an electrical field is built up between the electrodes of the electrostatic transducer, it is possible to transpose the capacity changes of the transducer into electrical voltage changes by means of a subsequently connected amplifier.
The electroacoustic properties of electrostatic microphone capsules are primarily dependent on the type of the acoustic excitation of the diaphragm. Known in the art are the so called pressure receivers, on the one hand, and the so called pressure-gradient receivers, on the other hand. The diaphragm of the first type is subjected to the field of sound only in one direction, i.e., the so called front direction; consequently, they react only to the changes of the air pressure due to the sound waves. Since the air pressure is a scalar quantity, microphones constructed in this manner have a spherical directional effect. In a pressure-gradient receiver, the diaphragm is subjected to the field of sound from both directions; consequently, the diaphragm is excited to movement by the pressure gradient prevailing at any given time between the front and rear sides of the diaphragm. Because of the travel time differences of the sound waves to the front side and the rear side, microphones constructed in this manner have a directional characteristic which, with a correct acoustic adjustment of the microphone capsule, may have any selected shape between an 8-shaped and kidney-shaped directional characteristic.
In the past, pressure-gradient microphone capsules were constructed in such a way that the front and rear sides of the diaphragm were arranged so as to coincide with the front and rear sound openings. This means that such microphone capsules have an axially symmetrical directional characteristic. FIG. 1 shows an electrostatic microphone capsule in accordance with the prior art. The capsule has a, front sound entry opening 21 and a rear sound entry opening 22.
It is now required more and more often that microphone capsules are mounted as much as possible flush in a flat and relatively large surface. This may be the roof of an automobile if a hands-free device for a mobile telephone or other communication device is to be provided in the automobile. However, microphone capsules according to the prior art are not capable of meeting this requirements and it is very difficult, expensive and technically cumbersome to mount the microphones in a flat surface. FIG. 2 of the drawing shows the usual mounting situation according to the prior art for a microphone capsule operating in accordance with the electrostatic principle. In order to make both sound entry openings accessible to the sound as it is required for the operation of the capsule, it is necessary to mount the capsule above the mounting surface, and not in the mounting surface as it is desired.
U.S. Pat. No. 5,226,076 A discloses mounting a capsule as illustrated in FIG. 2 in a housing which has two sound openings at one of its longitudinal walls, wherein one sound opening each is arranged on both sides of the capsule. This makes it possible to mount the housing in or behind a flat mounting surface. However, this solution has the disadvantage that it has a great structural height or depth because the axis of symmetry of the capsule extends parallel to the mounting plane and, therefore, the capsule has a high structural height. In addition, the assembly of the microphone including the capsule with its housing is complicated and expensive because the housing essentially is an added component which is completely separate from the finished microphone.
Because of the nature of a pressure receiver, microphone capsules with only one sound entry operating according to the principle of the pressure receiver can be integrated without problems in the mounting surface. However, since they have a spherical directional characteristic, it is then not possible to utilize the significant advantages of the directional characteristic. Because of their spherical directional characteristic, the pressure receivers are equally sensitive to useful sound and to interfering sound. Consequently, they are usually not used in a loud environment.